Micro light-emitting diode display driver architecture and pixel structure

ABSTRACT

Micro light-emitting diode display driver architectures and pixel structures are described. In an example, a driver circuit for a micro light emitting diode device includes a current mirror. A linearized transconductance amplifier is coupled to the current mirror. The linearized transconductance amplifier is to generate a pulse amplitude modulated current that is provided to a set of micro LEDs connected in parallel to provide fault tolerance architecture.

TECHNICAL FIELD

Embodiments of the disclosure are in the field of micro-LED displaysand, in particular, micro light-emitting diode display driverarchitectures and pixel structures.

BACKGROUND

Displays having micro-scale light-emitting diodes (LEDs) are known asmicro-LED, mLED, and μLED. As the name implies, micro-LED displays havearrays of micro-LEDs forming the individual pixel elements.

A pixel may be a minute area of illumination on a display screen, one ofmany from which an image is composed. In other words, pixels may besmall discrete elements that together constitute an image as on adisplay. These primarily square or rectangular-shaped units may be thesmallest item of information in an image. Pixels are normally arrangedin a two-dimensional (2D) matrix, and are represented using dots,squares, rectangles, or other shapes. Pixels may be the basic buildingblocks of a display or digital image and with geometric coordinates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a circuit conventionally used for drivingLED devices.

FIG. 2 is a plot of power efficacy as a function of current density fora green (InGaN/GaN) LED, in accordance with an embodiment of the presentdisclosure.

FIG. 3A is a block diagram of pulse amplitude modulation architecture,in accordance with an embodiment of the present disclosure.

FIG. 3B is a block diagram of a pixel circuit including a linearizedtransconductance amplifier, in accordance with an embodiment of thepresent disclosure.

FIG. 4 illustrates a circuit for implementing pulse amplitudemodulation, in accordance with an embodiment of the present disclosure.

FIG. 5 illustrates a schematic of micro LED or OLED displayarchitecture, in accordance with an embodiment of the presentdisclosure.

FIG. 6 illustrates a circuit compatible with IGZO-based TFT backplanes,in accordance with an embodiment of the present disclosure.

FIG. 7 illustrates a cross-sectional view of a pixel structure inaccordance with an embodiment of the present disclosure.

FIG. 8 a plot of emission patterns of a microlens-integrated micro LED,compared with a reference micro LED without a microlens, in accordancewith an embodiment of the present disclosure.

FIG. 9 includes a Table summarizing typical subpixel sizes for differentdevices, in accordance with an embodiment of the present disclosure.

FIG. 10 illustrates a cross-sectional view of a red-green-blue pixel (anRGB pixel) with three nanowire LEDs, in accordance with an embodiment ofthe present disclosure.

FIG. 11 is a top view schematic of a subpixel arrangement on a donorwafer, in accordance with an embodiment of the present disclosure.

FIG. 12 is a “zoom out” view of FIG. 11 shown as an array for multiplepixels, in accordance with an embodiment of the present disclosure.

FIG. 13A illustrates a cross-sectional view of a GaN nanowire based LEDhighlighting certain layers of the LED, in accordance with an embodimentof the present disclosure.

FIG. 13B illustrates a cross-sectional view of a micro-LED composed ofmultiple nanowire LEDs, in accordance with an embodiment of the presentdisclosure.

FIG. 13C illustrates a cross-sectional view of a GaN nanopyramid ormicropyramid based LED highlighting certain layers of the LED, inaccordance with an embodiment of the present disclosure.

FIG. 13D illustrates a cross-sectional view of a GaN axial nanowirebased LED highlighting certain layers of the LED, in accordance with anembodiment of the present disclosure.

FIG. 14 is a flow diagram illustrating an RGB display productionprocess, in accordance with an embodiment of the present disclosure.

FIG. 15 is a schematic illustration of a display architecture, inaccordance with an embodiment of the present disclosure.

FIG. 16 is an electronic device having a display, in accordance withembodiments of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

Micro light-emitting diode (LED) display driver architectures and pixelstructures are described. In the following description, numerousspecific details are set forth, such as specific material and structuralregimes, in order to provide a thorough understanding of embodiments ofthe present disclosure. It will be apparent to one skilled in the artthat embodiments of the present disclosure may be practiced withoutthese specific details. In other instances, well-known features, such assingle or dual damascene processing, are not described in detail inorder to not unnecessarily obscure embodiments of the presentdisclosure. Furthermore, it is to be understood that the variousembodiments shown in the Figures are illustrative representations andare not necessarily drawn to scale. In some cases, various operationswill be described as multiple discrete operations, in turn, in a mannerthat is most helpful in understanding the present disclosure, however,the order of description should not be construed to imply that theseoperations are necessarily order dependent. In particular, theseoperations need not be performed in the order of presentation.

Certain terminology may also be used in the following description forthe purpose of reference only, and thus are not intended to be limiting.For example, terms such as “upper”, “lower”, “above”, “below,” “bottom,”and “top” refer to directions in the drawings to which reference ismade. Terms such as “front”, “back”, “rear”, and “side” describe theorientation and/or location of portions of the component within aconsistent but arbitrary frame of reference which is made clear byreference to the text and the associated drawings describing thecomponent under discussion. Such terminology may include the wordsspecifically mentioned above, derivatives thereof, and words of similarimport.

One or more embodiments described herein are directed to devices andarchitectures for micro LED displays. To provide context, displays basedon inorganic micro LEDs (μLEDs) have attracted increasing attention forapplications in emerging portable electronics and wearable computerssuch as head-mounted displays and wristwatches. Micro LEDs are typicallyfirst manufactured on Sapphire or silicon wafers (for example) and thentransferred onto a display backplane glass substrate where on whichactive matrix thin-film transistors have been manufactured.

Micro LED displays promise 3×-5× less power compared to organic LED(OLED) displays. The difference would result in a savings in batterylife in mobile devices (e.g., notebook and converged mobility) and canenhance user experience. In an embodiment, micro LED displays describedherein consume two-fold less power compared to organic LED (OLED)displays. Such a reduction in power consumption may provide anadditional approximately 8 hours of battery life. Such a platform mayeven outperform platforms based on low power consumption centralprocessing units (CPUs). Embodiments described herein may be associatedwith one or more advantages such as, but not limited to, highmanufacturing yield, high manufacturing throughput (display per hour),and applicability for displays with a diagonal dimension ranging from 2inches to 15.6 inches.

In a first aspect of the present disclosure, display driverarchitectures using pulse amplitude modulation for micro LED displaysare described.

One or more embodiments are directed to a system or method for driving amatrix of micro light emitting diode devices (μLEDs) in an active-matrixdisplay. Displays based on inorganic micro LEDs (μLEDs) have attractedincreasing attention for applications in emerging portable electronicsand wearable computers such as head-mounted displays and wristwatches.Micro LED displays promise approximately 3× less power compared to OLEDdisplays. The reduction in power consumption saves battery life inmobile devices and can enhance a user experience. It is to beappreciated that driving architecture described herein may also be usedto drive micro OLED displays fabricated on complementary metal oxidesemiconductor (CMOS) backplanes for virtual reality displays.

To provide background context, FIG. 1 is a circuit diagram of a circuit100 conventionally used for driving LED devices. With reference to FIG.1, for analog driving, a traditional two-transistor one-capacitor (2T1C)analog drive pixel circuit employs a first transistor (T1) as a switch.When SCAN is low, T1 is open, so a second transistor (T2) passes acurrent to the LED depending on the voltage level of a Data line. Withreference again to FIG. 1, for digital driving, a traditionaltwo-transistor one-capacitor (2T1C) digital drive pixel circuit employsboth first and second transistors (T1 and T2) as switches. When SCAN islow, T1 is open, so T2 is turned on or off depending on the voltagelevel of a Data line. When SCAN is high, T2 is switched off, so thevoltage level is held in C_(S). The OLED pixel current has only twostates: on and off. The pulsing of OLED current can be modulated bycontrolling SCAN and Data line by varying either the pulse width ordensity.

It is to be appreciated that, given a size of a LED in the range of 5μm, the current required to drive a single LED for maximum luminance(e.g., 30-300 nits) is in the 1-100 nA range. Regarding state-of-the-artapproaches, with respect to analog driving, the power efficacy ofsolid-state μLEDs drops significantly at very low current density (e.g.,less than 1 A/cm²). The drop in efficacy can cause undesired high powerconsumption.

As an example of power efficacy versus current density, FIG. 2 is a plot200 of power efficacy as a function of current density for a green(InGaN/GaN) LED, in accordance with an embodiment of the presentdisclosure. Referring to plot 200 of FIG. 2, power efficacy peaks at acurrent density of approximately 1 A/cm². However, typical operatingconditions are current density are approximately 0.1 A/cm² for displayapplications. At such a low current level, the power efficacy is smallerthan its optimal (peak) value. If the brightness demands forcing acurrent density that is smaller than approximately 1 A/cm² in this case,the LED will operate under sub-optimal conditions (i.e. lower powerefficacy). Thus, potential brightness of the LED is not maximized.

Regarding state-of-the-art approaches, with respect to pulse widthmodulation digital driving, a false contour issue can be a major displayquality concern. For pulse density modulation (PDM) digital driving,with respect to high pixel per inch (PPI) resolution displays, the pulsewidth can be less than 10 ns. This is comparable to the μLED responsetime and the PDM scheme can cease to work.

In accordance with an embodiment of the present disclosure, addressingone or more of the above described issues with state-of-the-arttechnologies, described herein is the use of pulse amplitude modulationto drive micro LEDs to achieve targeted brightness and power efficiency.In an embodiment, a circuit described herein is implemented for pulseamplitude modulation. In a particular embodiment, a μLED array is drivenby a row and column driver. Each column driver has 8 bit SRAM and a 256Bit digital to analog convertor (DAC). The output of the DAC is a pulsewith an amplitude determined by the current density required to achievepeak power efficacy. The width of the pulse is a function of theintegrated current density needed by the micro LED to achieve a desiredgray level.

Advantages of implementing one or more embodiments described herein mayinclude one or more of, but are not limited to: (1) a very low “average”current can be passed to μLEDs but the input current pulse amplitude canbe large enough to improve circuit speed and operate at peak powerefficacy of micro LEDs; (2) all transistors in the driver circuit may beoperated in strong inversion operating regime, which is more stable andless vulnerable to variability; and/or (3) provision of aself-compensated circuit with respect to threshold voltage variation dueto process variations and/or transistor instability.

One or more embodiments provide multiple knobs to control micro LEDcurrent at the nano-amp (nA) level without sacrificing speed (e.g.,settling times) or display quality. Pulse amplitude modulation circuitarchitecture for pixel driving described herein may enable the removalof a limitation of pulse width associated with digital driving and, atthe same time, may reduce the power associated with static powerconsumption of the DAC in the analog driving circuit architecture for amicro OLED.

To provide further context, LED arrays produce their own light inresponse to current flowing through the individual elements of thearray. A variety of different LED-like luminescent sources have beenused for such displays. One or more embodiments described herein utilizeelectroluminescent materials in μLEDs made of, for example, GaN, InGaN,or AlInGaP materials. Electrically, such devices behave like diodes withforward “on” voltage drops ranging from 1.9 volts (V) to 3 V, dependingon the color.

Unlike liquid crystal displays (LCDs), μLEDs are current driven devices.However, they may be similarly arranged in a two-dimensional array(matrix) of elements to form a display. Active-matrix μLED displaystypically use current control circuits integrated with the displayitself, with one control circuit corresponding to each individualelement on the substrate, to create high-resolution color graphics witha high refresh rate. Such a structure results in a matrix of devices,where one (or more) device is formed at each point where a row overliesa column. There will generally be at least M×N devices in a matrixhaving M rows and N columns. Typical devices function like lightemitting diodes (LEDs), which conduct current and luminesce when voltageof one polarity is imposed across them, and block current when voltageof the opposite polarity is applied. To control such individual μLEDdevices located at the matrix junctions, it may be useful to have twodistinct driver circuits, one to drive the columns and one to drive therows. It is conventional to sequentially scan the rows (e.g.,conventionally connected to device cathodes) with a driver switch to aknown voltage such as ground, and to provide another driver to drive thecolumns (which are conventionally connected to device anodes). Inoperation, information is transferred to the matrix display by scanningeach row in sequence. During each row scan period, each column connectedto an element intended to emit light is also driven.

In accordance with one or more embodiments of the present disclosure. Apulse amplitude modulation driving scheme and circuit are described. Forexample, FIG. 3A is a block diagram 300 of pulse amplitude modulationarchitecture, in accordance with an embodiment of the presentdisclosure. Referring to the display system schematic of FIG. 3A, a μLEDarray 302 (such as an OLED or LED) is driven by a row driver 304 and acolumn driver 306. Each column driver 306 will has 8 bit SRAM 308 and a256 bit DAC or 10 bit PAM 310. The output of the DAC 310 is a pulsehaving an amplitude determined by the current density required toachieve peak power efficacy. The width of the pulse is a function of theintegrated current density needed by the micro LED to achieve a desiredgray level.

FIG. 3B is a block diagram of a pixel circuit including a linearizedtransconductance amplifier, in accordance with an embodiment of thepresent disclosure. Referring to FIG. 3B, a circuit 350 includes a pixelcircuit 352. Pixel circuit 352 includes a current mirror 354 and alinearlized transconductance amplifier 356. A pulsed current source 358is provided. Input data 360 is input to pixel circuit 352. Output data362 is output from pixel circuit 352 and used to drive one or more microLED devices 364.

FIG. 4 illustrates a circuit 400 for implementing pulse amplitudemodulation, in accordance with an embodiment of the present disclosure.The circuit 400 includes a current mirror 402 and a linearizedtransconductance amplifier 404. In one embodiment, the current mirror402 is based on two P-type transistors, as is depicted. In the pulseamplitude modulation circuit 400, an input voltage signal is driven by aDAC. The linearized transconductance amplifier 404 converts the voltageto current. At the bottom of circuit 400, the current itself getsswitched to generate a pulse amplitude modulated current (e.g., biascurrent 406) as a pulsed current source. The width of the pulse is fixedby the amount of current density needed for representing a Gray level 1.

Regarding circuit analysis, referring again to the pulse amplitudemodulation circuit 400 of FIG. 4, the following equations hold.

$\begin{matrix}{V_{a} = {{V_{i} - V_{{GS}\; 1}} = {V_{i} - V_{TH} - \sqrt{\frac{2\; I_{1}}{\mu \; C_{ox}n}}}}} & (1) \\{V_{b} = {{- V_{{GS}\; 2}} = {{- V_{TH}} - \sqrt{\frac{2\; I_{2}}{\mu \; C_{ox}n}}}}} & (2) \\{{I_{1} - I_{2}} = {2\left( {I_{3} - I_{4}} \right)}} & (3) \\{I_{3} = {m\mspace{11mu} \mu \mspace{11mu} {C_{ox}\left\lbrack {{\left( {V_{i} - V_{b} - V_{TH}} \right)V_{ab}} - {\frac{1}{2}V_{ab}^{2}}} \right\rbrack}}} & (4) \\{I_{4} = {m\mspace{11mu} \mu \mspace{11mu} {C_{ox}\left\lbrack {{\left( {0 - V_{a} - V_{TH}} \right)V_{ba}} - {\frac{1}{2}V_{ba}^{2}}} \right\rbrack}}} & (5) \\{I_{1} = {{{\frac{1}{2}I_{0}} + I_{3} - I_{4}} = {{\frac{1}{2}I_{0}} + x}}} & (6) \\{I_{2} = {{{\frac{1}{2}I_{0}} - I_{3} + I_{4}} = {{\frac{1}{2}I_{0}} - x}}} & (7)\end{matrix}$

Using equations (1)-(5), the following equation can be derived withoutany approximations.

$\begin{matrix}{{I_{3} - I_{4}} = {\frac{n\; m}{n + {4\; m}}\sqrt{\frac{2\; \mu \; C_{ox}}{n}}\left( {\sqrt{I_{1}} + \sqrt{I_{2}}} \right)V_{i}}} & (8)\end{matrix}$

Using equations (6) and (7) into (8), the following is obtained.

$\begin{matrix}{x = {\frac{n\; m}{n + {4\; m}}\sqrt{\frac{2\; \mu \; C_{ox}}{n}}\left( {\sqrt{\frac{I_{0}}{2} + x} + \sqrt{\frac{I_{0}}{2} - x}} \right)V_{i}}} & (9)\end{matrix}$

Solving the quadratic equation in x, the following equation is derived.

$\begin{matrix}{I_{out} = {{I_{1} - I_{2}} = {\frac{4\; m}{1 + {4\; m\text{/}n}}\sqrt{\frac{\; {\mu \; C_{ox}}}{n}}\sqrt{I_{0}}V_{i}}}} & (10)\end{matrix}$

Equation (10) is valid when the following condition is satisfied.

$\begin{matrix}{V_{i}{\frac{n + {4\; m}}{n\; m}\sqrt{\frac{n}{4}}\sqrt{\frac{I_{0}}{\mu \; C_{ox}}}}} & (11)\end{matrix}$

Combining equations (10) and (11) the following condition is determined.

I ₀ >>I _(out)/2=5I _(out)  (12)

In an embodiment, for a driving method, each of the gray levels isrepresented by a specific voltage (V_(i)) in equation (10). If VIcorresponds to the lowest gray level, and V10 corresponds to 1024^(th)gray level (e.g., in a 10-bit architecture), the DAC should providevoltage levels with resolution equal to (V10-V1)/1024 volts.

The highest gray level current is given by the following.

$\begin{matrix}{I_{{GL},\max} = {{I_{out}\frac{t_{p}}{1\text{/}\left( {N_{r}f} \right)}} = {\frac{I_{0}}{5}t_{p}N_{r}f}}} & (13)\end{matrix}$

Thus, the current I₀ should be a pulse with fixed width (t_(p)) andamplitude that is equal to 5 I_(GL,max) ((t_(p)·f·N_(r)). Here,I_(GL,max) is the micro LED current that corresponds to a highest graylevel, f is the frame rate (e.g. 120 Hz), and N_(r) is the number ofrows in the active-matrix. The pulse amplitude should also equal to thecurrent at which the micro LED power efficacy peaks (J_(p)L²) whereJ_(p) is the current density at peak power efficacy, and L is the sizeof the micro LED.

Therefore, the following holds.

J _(p) L ²=5I _(GL,max)/(t _(p) ·f·N _(r))  (14)

The pulse width can thus be determined to be the following.

t _(p)=5I _(GL,max)/(f·N _(r) ·J _(p) ·L ²)  (15)

In equation (15), I_(GL,max) is the current corresponding to the highestgray level brightness. If it is assumed that I_(GL,max)=10 nA, f=120 Hz,N_(r)=1440, J_(p)=1 A/cm², and L=5 μm, then t_(p)≈115 μs.

In an embodiment, for pulse amplitude modulation, circuit 400 shown inFIG. 4 performs pulse amplitude modulation according to the following.

$\begin{matrix}{I_{out} = {\frac{4\; m}{1 + {4\; m\text{/}n}}\sqrt{\frac{\; {\mu \; C_{ox}}}{n}}\sqrt{I_{0}}V_{i}}} & (16)\end{matrix}$

In equation (16), the pulse current (I₀) is modulated by the input DATAvoltage V_(i). The pulse height and pulse width t_(p) are designed asdescribed above.

In a second aspect of the present disclosure, an indium gallium zincoxide (IGZO) thin film transistor (TFT) or IGZO-type TFT backplane fordigital driving of micro LED displays is described.

To provide context, low temperature polysilicon (LTPS) backplanes aretypically used in OLED displays, including those that are manufacturedon flexible substrates. However, LTPS backplanes are expensive, in part,due to the large number of masks. Additionally, LTPS backplanes do not,at present, scale beyond sixth generation (1500×1850 cm) factories,thereby limiting cost reduction opportunities via scalability to largerbackplanes which scale to tenth generation (2940×3320 cm) displayfactories.

In accordance with one or more embodiments of the present disclosure, asan alternative to an LTPS backplane, an IGZO TFT or IGZO-type TFTbackplane is used. It is to be appreciated, however, that IGZO TFTs maybe prone to threshold voltage shift under prolonged electrical stress.Additionally, current-voltage characteristics IGZO TFTs may change,which may alter the data current by more than 60% in a conventionalpixel circuit. By contrast, in an embodiment, pixel circuits are usedthat provide a bulwark against threshold voltage shift in a μLEDdisplay. This enables the use of IGZO TFT or IGZO-type TFT backplanes.Such IGZO TFT or IGZO-type TFT backplanes may be more cost effectivethan LTPS TFT backplanes.

To provide further context, IGZO TFT backplanes used with conventionalcircuits based on two transistors and one capacitor circuit (e.g., asdescribed above with respect to FIG. 1) do not provide a requiredcompensation of the threshold voltage shift that results from prolongedusage. By contrast, in accordance with one or more embodiments describedherein, a μLED or μOLED array is driven by a row and column driver. Eachcolumn driver has 8 bit SRAM and a 256 Bit DAC. The output of the DAC isthe pulse with an amplitude determined by the current density requiredto achieve peak power efficacy. The width of the pulse is a function ofthe integrated current density needed by the micro LED to achieve adesired gray level and brightness.

In an embodiment, pixel circuits based on N-channel IGZO TFTs are usedprovide a bulwark against threshold voltage shift in a micro LED ormicro OLED displays. The use of such N-channel IGZO TFTs enables the useof IGZO TFT or IGZO-type TFT backplanes that are cheaper compared toLTPS TFT backplanes. An exemplary IGZO-TFT pixel circuit design isdescribed below in association with FIG. 6. An exemplary IGZO-TFT pixelstructure that may be included in an LED display is described below inassociation with FIG. 7.

In an embodiment, advantages of implementing one or more embodimentsdescribed herein may include, but need not be limited to, one or more of(1) low manufacturing cost, (2) reliable displays, and/or (3) digitaldriving which provides low power consumption for micro LEDs. It is to beappreciated that demand for low power in consumer electronic devices hasincreased dramatically in the past ten years due to limited batterylifespan. One of the components with the highest percentage of totalenergy consumption, and therefore a suitable candidate for improvement,is the display. The developments of low power displays are becoming ahigh priority for the consumer electronics industry. Micro LED (μLED)display is a type of emissive display technology that uses a matrix ofindividually-switched self-illuminating inorganic diodes that can becontrolled and lit without a master backlight. Inorganic μLEDs have anumber of potential advantages over organic LEDs (OLEDs) for displayapplications including high brightness, longer lifecycle, andimperviousness to image sticking and burn in. Typically, in μLEDdisplays, a desired color and luminance value are created from variouscombinations of three colors of light emitting elements (red, green andblue).

As an exemplary display architecture, FIG. 5 illustrates a schematic ofmicro LED or OLED display architecture, in accordance with an embodimentof the present disclosure. Referring to FIG. 5, a micro LED or OLEDdisplay 500 includes a backplane 502 having pixel circuits 504 thereon.An insulator 506 is over the pixel circuits 504. Micro LED layers 508are included over the insulator 506. A transparent electrode 510 is overthe micro LED layers 508. In one embodiment, pixel circuits 504 aredescribed herein that are compatible with IGZO TFTs and IGZO-type TFTs.

To provide further context, since amorphous Indium-Gallium-Zinc-Oxide(a-IGZO) has been successfully employed in organic light-emitting diode(OLED) TV products, requirements of higher mobility and improvedstability has become more stringent as OLED TVs move toward higherresolution, higher frame rate, higher brightness and longer lifetime.Manufacturability of amorphous oxide TFT on Generation 8 glass or evenlarger sizes of glass, which is a major advantage over low temperaturepoly-Si (LTPS) TFTs, has accelerated to develop more stable and highermobility oxide TFTs. The improvements apply to not only OLED TVs butalso to other applications that LTPS TFTs have dominated up to now. TheIGZO TFT performance stability requirement becomes more stringent indisplay applications with higher resolution, higher frame rate, higherbrightness, and longer target product lifetime. Current-driving TFTs inan OLED pixel are often under the influence of positive gate-biastemperature stress (PBTS). Under the influence of PBTS, the thresholdvoltage (VT) shifts in the positive direction. The physical origin ofPBTS instability has been classified largely by a combination of (1)trapping of electrons in the gate insulator and (2) change in defectstates in the IGZO channel region.

It is to be appreciated that process technology changes implemented toaddress the above issue can be complex and expensive, leading to highermanufacturing cost for IGZO-based TFT backplanes. As such, the apparentcost advantage of IGZO TFT compared to LTPS TFT backplanes may bereduced if additional or alternative processing is used that isrelatively expensive. Alternatively, cost effective solutions mayinclude the use of a compensation circuit design in order to meet bothcost and reliability targets. Circuits described in accordance withembodiments herein may be implemented to provide a cost effectivesolution to meet lifetime requirements.

In accordance with one or more embodiments of the present disclosure,designs for pixel circuits driving OLEDs or micro LEDs are describedbased on circuits manufactured with IGZO TFTs. Designs described hereinmay address key challenges presently associated with in using IGZO TFTs,e.g., the threshold voltage shift under positive bias stress and exampleof which may be prolonged use under normal operating conditions of thedisplay.

FIG. 6 illustrates a circuit 600 compatible with IGZO-based TFTbackplanes, in accordance with an embodiment of the present disclosure.The circuit 600 includes a current mirror 602 and a linearizedtransconductance amplifier 604. In one embodiment, the current mirror602 is based on four N-type transistors, as is depicted. In the circuit600, an input voltage signal is driven by a DAC. The linearizedtransconductance amplifier 604 converts the voltage to current. At thebottom of circuit 600, the current itself gets switched to generate apulse amplitude modulated current (e.g., bias current 606) as a pulsedcurrent source. The width of the pulse is fixed by the amount of currentdensity needed for representing the maximum Gray level. Regardingcircuit analysis, referring again to the pulse amplitude modulationcircuit 600 of FIG. 6, the above equations described in association withcircuit 400 also hold for circuit 600.

In an embodiment, circuit 600 is referred to as an IGZO TFT pixeldriving circuit. It is to be appreciated that in circuit 600 as shown inFIG. 6, in accordance with a specific embodiment, two micro LEDs areconnected in parallel. However, it is to be appreciate that, in general,the number of micro LEDs driven by a circuit 600 may be one or more thantwo. As described above with respect to FIG. 3A, and as applicable forthe architectures represented by circuit 600, a μLED array may be drivenby a row and column driver. In one embodiment, each column driver mayhave 8 bit SRAM and a 256 Bit DAC. The output of the DAC is the pulsehaving an amplitude determined by the current density required toachieve peak power efficacy. The width of the pulse is a function of theintegrated current density needed by the micro LED to achieve a desiredgray level.

In a third aspect of the present disclosure, pixel structures forenhancing light extraction and controlling viewing angle for micro LEDdisplays are described.

In accordance with one or more embodiments of the present disclosure, adevice and method for fabricating full-color micro light emitting diode(μLED) displays with controllable viewing angle are described. It is tobe appreciated that μLED displays promise 3×-5× less power compared toOLED displays. The result may translate into a savings in battery lifein mobile devices (e.g., notebooks and converged mobility) and mayenhance a user experience. Improvements in structures and fabricationprocesses are needed to realize low power, full color μLED displays,such as for displays described above in association with FIG. 5.

To provide context, state-of-the-art approaches to addressing one ormore of the above issues include the use of a backplane having a“subpixel bank” designed to reflect light from a micro LED and helpproduce a desired viewing angle. However, drawbacks to implementing suchan architecture may include high manufacturing cost, and the difficultyassociated with the need for a special backplane design that isdifferent from that used for OLED displays. Such a design, for example,may not be compatible with a nanowire micro LED design, such asdescribed below, that can otherwise enable low display production costand improve power reduction.

In accordance with one or more embodiments described herein, a subpixelbank layer is used on a display backplane to control a viewing angle.The subpixel bank layer thickness and angle can be used to adjust thedisplay viewing angle. In the arrangement, μLED dies are positioned intothe pixel bank by a transfer tool and connected by solder reflow orother processes. It is to be appreciated that more than one μLED of asame color can be placed in each bank if redundancy is required tomitigate risks of dead/malfunctioning pixels. In an embodiment, eachpixel includes three subpixels: Red, Green, Blue (R,G,B). It is to beappreciated that other configurations with more subpixels are possible,for example R,G,B+Yellow. The physical size of the actual micro LED canbe significantly smaller than that of the subpixel. In one embodiment,the pixel bank can occupy less than the available space allowed by thepixel pitch. It is to be appreciated that the micro LED can have variousshapes, such as, but not limited to, square, rectangular or circular.

In an embodiment, the maximum dimension of the micro LED emitter islimited by the subpixel pitch, which may be one third of the pixel pitchin one direction. In an actual device, subpixel banks may be separatedby at least 1-2 μm and the micro LED is smaller than the bank by atleast 1 μm in order to allow for positioning tolerance. As a result, inaccordance with one embodiment, the minimum micro LED dimension istypically less or equal to the subpixel pitch minus about 1.5-2 μm.

In accordance with one or more embodiments described herein, a microlensis implemented on top of each micro LED subsequent to transferring themicro LED from a donor wafer to a “standard” backplane. The microlensmay assist to provide improved light extraction and control of a viewingangle. In an embodiment, such a microlens is fabricated using thermalreflow of photoresist, self-assembling of microspheres as microlenses,or ink-jet processing.

As an exemplary pixel architecture, FIG. 7 illustrates a cross-sectionalview of a pixel structure in accordance with an embodiment of thepresent disclosure.

Referring to FIG. 7, a pixel structure 700 includes a backplane 701. Thebackplane 701 includes a glass substrate 702 having an insulating layer704 thereon. Pixel thin film transistor (TFT) circuits 706 are includedin and on the insulating layer 704. Each of the pixel TFT circuits 706includes gate electrodes 707A, such as metal gate electrodes, andchannels 707B. A portion of the insulating layer 704 may act as a gatedielectric for each of the pixel TFT circuits 706.

Referring again to FIG. 7, the pixel structure 700 includes a frontplane 708 on the backplane 701. The front plane 708 includes LEDs in adielectric layer 710, such as a carbon-doped oxide layer. In anexemplary embodiment, three micro LEDs 712, 714 and 716 are included. Ina particular embodiment, micro LED 712 is a red micro LED, micro LED 714is a green micro LED, and micro LED 716 is a blue micro LED. It is to beappreciated that other arrangements may be used, including variation innumber and/or colors of micro LEDs included.

Referring again to FIG. 7, the front plane 708 includes a transparentconducting oxide layer 718 as a cathode of the pixel structure 700. Amask layer 722, such as a layer of CrO₂, is on the conducting oxidelayer 718. Microlenses, such as microlenses 713, 715 and 717 (associatedwith micro LEDs 712, 714 and 716, respectively), are disposed inopenings in the mask layer 722 over an associated micro LED. In anembodiment, the arrangement provides for a collimated radiation pattern720.

In an embodiment, each of the TFT circuits 706 is a circuit such ascircuit 400 or 600 described above. Embodiments described herein may bebased only on the back plane 701 described above. Embodiments describedherein may be based only on the front plane 708 described above.Embodiments described herein may be based on a front plane that does notinclude microlenses.

In an embodiment, an LED pixel structure 700 with integrated microlenseson each micro LED can (1) enhance extraction efficiency, and (2) tunethe viewing angle per each display application. Since the radiationpattern from a nanowire LED is almost collimated, the image and colorquality may be readily preserved.

In an embodiment, the TFTs 706 are IZGO TFTs or IGZO-type TFTs, wherethe channel 707B of each of the TFTs 706 includes a semiconducting oxidematerial. In an embodiment, the semiconducting oxide material is an IGZOlayer that has a gallium to indium ratio of 1:1, a gallium to indiumratio greater than 1 (e.g., 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, or10:1), or a gallium to indium ratio less than 1 (e.g., 1:2, 1:3, 1:4,1:5, 1:6, 1:7, 1:8, 1:9, or 1:10). A low indium content IGZO may referto IGZO having more gallium than indium (e.g., with a gallium to indiumratio greater than 1:1), and may also be referred to as high galliumcontent IGZO. Similarly, low gallium content IGZO may refer to IGZOhaving more indium than gallium (e.g., with a gallium to indium ratioless than 1:1), and may also be referred to as high indium content IGZO.In another embodiment, the semiconducting oxide material is or includesa material such as tin oxide, antimony oxide, indium oxide, indium tinoxide, titanium oxide, zinc oxide, indium zinc oxide, gallium oxide,titanium oxynitride, ruthenium oxide, or tungsten oxide.

In an embodiment, the semiconducting oxide material is an amorphous,crystalline, or semi crystalline oxide semiconductor, such as anamorphous, crystalline, or semi crystalline oxide semiconductor IGZOlayer. The semiconducting oxide material may be formed using alow-temperature deposition process, such as physical vapor deposition(PVD) (e.g., sputtering), atomic layer deposition (ALD), or chemicalvapor deposition (CVD). The ability to deposit the semiconducting oxidematerial at temperatures low enough to be compatible with back-endmanufacturing processes represents a particular advantage. Thesemiconducting oxide material may be deposited on sidewalls orconformably on any desired structure to a precise thickness, allowingthe manufacture of transistors having any desired geometry.

In an embodiment, gate electrode 707A includes at least one P-type workfunction metal or N-type work function metal. For a P-type transistors,metals that may be used for the gate electrode 707A may include, but arenot limited to, ruthenium, palladium, platinum, cobalt, nickel, andconductive metal oxides (e.g., ruthenium oxide). For an N-typetransistor, metals that may be used for the gate electrode 707A include,but are not limited to, hafnium, zirconium, titanium, tantalum,aluminum, alloys of these metals, and carbides of these metals (e.g.,hafnium carbide, zirconium carbide, titanium carbide, tantalum carbide,and aluminum carbide).

Advantages of implementing embodiments described herein may include, butneed not be limited to, providing a path to low manufacturing cost,which is accomplished by transferring the red-green-blue micro LEDpixels in one pass from silicon wafer to a “standard” display backplane.Improved light extraction may be realized with the use of microlenses. Asignificant reduction in reflectivity may be achieved to provideenhanced transmission, which serves to assist with light extraction fromthe micro devices. A tunable radiation pattern may be accomplished bydepositing micro lenses on red, green, and blue micro LEDs aftertransferring to a backplane and depositing the transparent conductingelectrode (e.g., indium tin oxide (ITO)) to form the common cathode forall micro LEDs.

As an exemplary radiation pattern, FIG. 8 a plot 800 of emissionpatterns of a microlens-integrated micro LED 804, compared with areference micro LED 802 without a microlens, in accordance with anembodiment of the present disclosure. Plot 800 provides an example ofhow a radiation pattern may be tuned using microlens integration onmicro LEDs. In an embodiment, power reductions with micro LED displaysmay be achieved by the fabrication of LEDs with high power efficaciesfor the three color LED emitters. In one such embodiment, the use of acorresponding microlens provides for LEDs with high power efficacies.

Embodiments described herein may be implemented to enable large scaleμLED display manufacturing that brings together three major separatetechnologies and supply chain bricks: (1) micro LED manufacturing, (2)display manufacturing, and (3) transfer technology tool manufacturing.In a typical display, each pixel is constituted of Red, Green and Blue(RGB) subpixels controlled independently by a matrix of transistors. Theidea behind μLED displays is to use individual, small LED chips as thesub-pixel. Unlike OLEDs, inorganic LED require high processingtemperatures (e.g., greater than 1000° C.) and cannot be “grown” andpatterned directly on top of the transistor matrix. In most cases, themicro LED chips are therefore manufactured separately then positionedand connected to the transistor matrix via a pick and place process.Many companies and research organizations are currently working on μLEDdisplays. However, volume production at costs compatible with theapplications still face multiple engineering and manufacturingchallenges. Such challenges include: LED epitaxy quality andhomogeneity, efficiency of very small μLEDs, sidewall effects, massivelyparallel chip transfer technologies (e.g. pick and place) with positionaccuracy and high throughput, cost, handling of small die, etc.,interconnects, color conversion, defect management, supply chain, andcost of production.

It is to be appreciated that due to the inorganic nature of the emittingmaterials of micro LEDs versus OLEDs, the efficiency and narrow emissionbands of μLEDs also offer the prospect of significantly improvedperformance in terms of: energy consumption, color gamut, brightness,contrast (High Dynamic Range), long lifetime and environmental stability(not sensitive to air, moisture), and compatibility with flexiblebackplane technologies to enable curved or flexible displays. Inaddition, μLEDs can deliver extremely high pixel density (up to 5000PPI) which, along with very high brightness, make them ideal forapplications such as Augmented Reality (AR) or Head Up Displayprojectors.

In accordance with an embodiment of the present disclosure, for adisplay structure and backplane, the size of microlens is approximatelythe same as the size of the subpixel, e.g., as is shown in FIG. 7. Inone embodiment, the RGB pixels are built directly upon the backplanethat includes the transistor and capacitors to drive each individualsubpixel. A micro LED is current driven and can use standard TFT LTPS ormetal-oxide based backplane manufactured for the OLED industry on largedimension glass or flexible polymer substrates. In another embodiment,IGZO TFTs are used. For displays with very high pixel density, standardlithography and integration levels of TFT fabs may not be insufficient.FIG. 9 includes a Table 900 summarizing typical subpixel sizes fordifferent devices, in accordance with an embodiment of the presentdisclosure.

In accordance with one or more embodiments of the present disclosure, ananowire LED structure has a superlambertian radiation pattern due tothe LED contact structure. An exemplary LED structure is described belowin association with FIG. 10. In one embodiment, light is created in thequantum well of the nanowire structure and reflected by a metalelectrode.

As an exemplary structure, FIG. 10 illustrates a cross-sectional view ofa red green blue pixel (an RGB pixel) with three nanowire LEDs, inaccordance with an embodiment of the present disclosure. Referring toFIG. 10, although shown as three different color micro-LEDs across(e.g., blue, green, red from left-right), the three are shown in thismanner for illustrative purposes only. It is to be appreciated that fora pixel such as a 2×2 pixel element, only two micro LEDs would beviewable for a given cross-section. It is to be appreciated that avariety of arrangements of micro LEDs may be suitable to make a singlepixel. In one embodiment, three micro LEDs are arranged side-by-side, asdepicted in FIG. 10. In another embodiment, four micro LEDs are arrangeda 2×2 arrangement. In another embodiment, nine micro LEDs are arranged a3×3 arrangement (three red micro LEDs, three green micro LEDs, and threeblue micro LEDs), etc. It is to be appreciated that a micro LED iscomposed of an array of nanowire LEDs. The number of nanowire LEDs perone micro LEDs is at least one. For example, a 10 micron×10 micron microLED may be composed of 90 nanowire LEDs connected in parallel to emitlight of a specific color. It is further to be appreciated that, withrespect to FIG. 10, the micro LEDs are represented by one nanowire eachfor illustrative purposes. This in general is not the case. Typically,one micro LED will be composed of more than one nanowire LED. Also, inFIG. 10, one example arrangement is shown. That is, the three colors areadjacent to each other. However, in some cases, the micro LEDs ofdifferent colors are separated on the source wafer by a distance thatmay be half of the display pixel pitch, for example.

With reference again to FIG. 10, in a particular embodiment, a sourcemicro LED wafer 1000 (such as a silicon wafer) has “RGB Chips”monolithically grown thereon. The silicon wafer 1000 is first coatedwith an aluminum nitride (AlN) buffer layer 1002, e.g., having athickness of approximately 50 nanometers. The AlN buffer layer 1002 mayhave a bandgap of about 6 eV and may be transparent to infraredradiation. A metal-based nucleation layer (MNL) 1004 is then depositedon the AlN buffer layer 1002. The MNL 1004 may have a thickness in therange of 30-100 nm and may be crystalline or polycrystalline. A siliconnitride mask 1006 is then deposited on the MNL. Lithography may then beused to open apertures in the silicon nitride mask 1006 mask withdiameters carefully chosen to accommodate the subsequent formation ofLEDs that emit red, green, and blue colors. N-type GaN nanowire coresare then grown, e.g., by metal organic chemical vapor deposition(MOCVD), as seeded from the MNL 1004. The nanowire cores may havediameters in the range 50 nm to 250 nm.

Referring again to FIG. 10, indium gallium nitride (InGaN) shells 1010are grown around the GaN cores 1008, e.g., using MOCVD. The amount ofindium in the InGaN shells 1010 depends on the GaN core diameter. In anembodiment, smaller core diameter result in the growth of InGaN shellswith smaller indium content. Larger core diameters result in the growthof InGaN shells with larger indium content. For blue (B) color emission,the indium content is approximately 20%. For green (G) color emission,the indium content is approximately 30%. For red (R) color emission, theindium content is approximately 40%. A p-type GaN cladding layer 1012may then be formed around the InGaN shells 1010, e.g., using MOCVD. Thecore-shell nanowires are the covered by an insulating material layer1014, e.g., a silicon oxide (SiOx) layer. A lithography and etch maythen be used to expose the p-GaN cladding layers 1012 for all colorcore-shell nanowire structures. Atomic layer deposition may then be usedto conformally deposit a metal layer 1016 on the p-GaN cladding layers1012. A metal fill process may then be performed to fill in contactmetals 1018 for the micro LED structures.

Referring more generally to FIG. 10, a semiconductor structure includesa silicon wafer 1000 and plurality of pixel elements 1050. Each of thepixel elements 1050 includes a first color nanowire LED, a second colornanowire LED (the second color different than the first color), and apair of third color nanowire LEDs (the third color different than thefirst and second colors). A continuous insulating material layer 1014 islaterally surrounding the first color nanowire LED, the second colornanowire LED, and the pair of third color nanowire LEDs. Adjacent pixelelements are separated from one another by a trench 1020 betweencorresponding continuous insulating material layers 1014. It is to beappreciated that more than three colors may be fabricated. For example,structures may be fabricated for red, green, yellow or blue emission. Inanother example, structures may be fabricated for red, orange, green, orblue emission.

In an embodiment, for each of the pixel elements 1050, the first coloris red, the second color is green, and the third color is blue. Inanother embodiment, for each of the pixel elements 1050, the first coloris red, the second color is blue, and the third color is green. Inanother embodiment, for each of the pixel elements 1050, the first coloris blue, the second color is green, and the third color is red. In anembodiment, for each of the pixel elements 1050, the first colornanowire LED, the second color nanowire LED, and the pair of third colornanowire LEDs have a 2×2 arrangement. In another embodiment, a structurereferred to as “monolithic blue and green only” may be fabricated. Insuch a case, three times as many blue micro LEDs as the green micro LEDsare fabricated. Then, after transfer of the blue and greed micro LEDs tothe display backplane (at one shot of transfer), quantum dots are addedon some of the blue micro LEDs to convert that blue to red color.

In an embodiment, an above described microlens used together with an LEDstructure of the type described in association with FIG. 10 enablesrealizing a wider viewing angle and improving the light extractionefficiency. In particular, light is generated in a InGaN/GaN quantumwell and reflected by a contact metal. The radiation pattern is narrowerthan Lambertian. By integrating meta-lenses on the bottom of each microLED (e.g., with red, green, or blue colors) a radiation pattern can begenerated that meets the viewing angle required by the device underconsideration.

FIG. 11 is a top view schematic of a subpixel arrangement on a donorwafer, in accordance with an embodiment of the present disclosure. Forexample, a red block 1102 (shown to have groups 1102A, 1102B and 1102C),a green block 1104 (shown to have groups 1104A, 1104B and 1104C) and ablue block 1106 (shown to have groups 1106A, 1106B and 1106C) includesrespective red, green, and blue subpixels separated by approximately onethird of pixel pitch to guarantee no color mixing or bleeding whentransferred to the display backplane. A “zoom out” is shown as array1200 in FIG. 12 for multiple pixels, in accordance with an embodiment ofthe present disclosure.

In accordance with one or more embodiments of the present disclosure,addressing both cost and defectivity requirements, monolithic red, greenand blue pixels are manufactured on a wafer and then transferred, asopposed to transferring individual micro LEDs with different colors fromthree separate source wafers sequentially. As described herein, sourcewafers are fabricated having individual red green blue (RGB) pixels(chips) thereon. Wafer-to-wafer bonding equipment and processtechnologies are then implemented to transfer micro LEDs from a sourcewafer to a target display backplane substrate, either directly orthrough an intermediate carrier plate. Thus, it is to be appreciatedthat typically three colors are transferred at the same time. It is notnecessarily the case that “one RGB pixel” is transferred. Rather, it maybe the case that one “whole” pixel is transferred. In another case, red,green, and blue micro LEDs are spaced appropriately on the wafer suchthat when they are transferred to the display backplane, they will landon pre-designated contact pads that may be separated by half of thepixel pitch or one quarter of the pixel pitch or other similar largeenough spacing to prevent color bleeding.

To provide further context for embodiments described herein, majorfactors driving the growth of the GaN semiconductor device industryinclude the vast addressable market for GaN in consumer electronics andautomotive, wide bandgap property of GaN material encouraging innovativeapplications, success of GaN in RF power electronics, and increasingadoption of GaN RF semiconductor device in military, defense andaerospace applications. GaN LEDs are widely used in laptop and notebookdisplay, mobile display, projectors, televisions and monitor, signs andlarge displays, etc. The market for GaN-based power drives is expectedto grow significantly during the forecast period attributed to itssuperior features such as minimum power loss, high-speed switchingminiaturization, and high breakdown voltage as compared with thesilicon-based power devices.

FIG. 13A illustrates a cross-sectional view of a GaN nanowire based LEDhighlighting certain layers of the LED, in accordance with an embodimentof the present disclosure. In the exemplary embodiment of FIG. 13A, anLED 1300 includes an n-type GaN nanowire 1302 above a substrate 1304,which may be a Si(001) substrate. An intervening nucleation layer 1306has an opened mask layer 1307 thereon. An active layer 1308/1310 (whichmay be a single active layer replacing 1308/1310) is included on then-type GaN nanowire 1302. In a particular embodiment, anIn_(0.2)Ga_(0.8)N shell “buffer” layer 1308 is included on the n-typeGaN nanowire 1302, and an active In_(0.4)Ga_(0.6)N layer 1310 isincluded on the In_(0.2)Ga_(0.8)N shell “buffer” layer 1308. In one suchembodiment, the In_(0.4)Ga_(0.6)N layer 1310 emits red color (e.g.,having a wavelength in the range of 610-630 nanometers). A p-GaN orp-ZnO cladding layer 1312 is included on the active layer 1308/1310.

In another such embodiment, following the fabrication of an orderedn-type In_(x)Ga_(1-x)N nanowire array with x in the range of 0.15-0.25,the remainder of the LED structure is grown radially around thenanowires. An In_(y)Ga_(1-y)N layer is on the In_(x)Ga_(1-x)N nanowires(and may be included in a set of In_(y)Ga_(1-y)N/GaN multi-quantum well(MQW) active layers) with y in the range of 0.4-0.45. An undoped GaNlayer and/or AlGaN electron blocking layer may be included as the nextouter layer. Finally, a p-type GaN (or p-type ZnO) cladding layer may beincluded.

FIG. 13B illustrates a cross-sectional view of a micro-LED composed ofmultiple nanowire LEDs, in accordance with an embodiment of the presentdisclosure. In the exemplary embodiment of FIG. 13B, a micro-LED 1320includes an n-GaN nano-column 1322 above a substrate 1324, which may bea Si(001) substrate. An intervening nucleation layer 1326 is includedbetween the n-GaN nano-column 1322 and the substrate 1324. An InGaN/GaNmulti-quantum well device (MQD) stack 1328 is included on the n-GaNnano-column 1322. A p-GaN layer 1330 is on the multi-quantum well device(MQD) stack 1328. A transparent p-electrode 1332 is included on thep-GaN layer 1330.

It is to be appreciated that foundational geometries other than theabove described nanowires may be used for LED fabrication. For example,in another embodiment, FIG. 13C illustrates a cross-sectional view of aGaN nanopyramid or micropyramid based LED highlighting certain layers ofthe LED, in accordance with an embodiment of the present disclosure. Inthe exemplary embodiment of FIG. 13C, an LED 1340 includes an n-GaNnanopyramid 1342 above a substrate 1344, which may be a Si(001)substrate. An intervening nucleation layer 1346 has an opened mask layer1347 thereon. An InGaN layer 1348 is included on the GaN nanopyramid1342. A p-GaN or p-ZnO cladding layer 1352 is included on the InGaNlayer 1348. It is to be appreciated that a micro LED may be composed ofmultiple nanopyramids connected in parallel. For example, a 5 um×5 ummicro LED may be composed of 20 nanopyramids.

In another embodiment, FIG. 13D illustrates a cross-sectional view of aGaN axial nanowire based LED highlighting certain layers of the LED, inaccordance with an embodiment of the present disclosure. In theexemplary embodiment of FIG. 13D, an LED 1360 includes an n-GaN axialnanowire 1362 above a substrate 1364, which may be a Si(001) substrate.An intervening nucleation layer 1366 has an opened mask layer 1367thereon. An InGaN layer 1368 is included on the GaN axial nanowire 1362.A p-GaN or p-ZnO cladding layer 1372 is included on the InGaN layer1368.

In another aspect, FIG. 14 is a flow diagram 1400 illustrating an RGBdisplay production process, in accordance with an embodiment of thepresent disclosure. Referring to flow diagram 1400, at operation 1402,an Si wafer has a nucleation layer formed thereon, such as a patternedconductive/dielectric nucleation layer. At operation 1404, sub 100 nmlithography is used to pattern a layer on the nucleation layer, or topattern the nucleation layer. At operation 1406, nanowire growth isperformed on the nucleation layer, e.g., by epitaxial deposition. Atoperation 1408, a backplane is introduced into the micro LED assemblyprocess. At operation 810, driver electrons are fabricated. At operation1412, display assembly is performed to finally provide a display.

FIG. 15 is a schematic illustration of a display architecture, inaccordance with an embodiment of the present disclosure. Referring toFIG. 15, micro LEDs 1502 are arranged in a matrix. The micro LEDs aredriven through “Data Driver” 1504 and “Scan Driver” 1506 chips. Thinfilm transistors 1508 are used to make “pixel driver circuits” 1510 foreach micro LED. In an embodiment, the micro LEDs are fabricated on asilicon wafer then transferred to a glass substrate called “backplane”where the “pixel driver circuits” 1510 have been fabricated using thinfilm transistors. Although represented simplistically in FIG. 15, it isto be appreciated that the pixel driver circuits 1510 may be or includea driver circuit such as circuit 400 or circuit 600 described herein.

FIG. 16 is an electronic device having a display, in accordance withembodiments of the present disclosure. Referring to FIG. 16, anelectronic device 1600 has a display or display panel 1602 with amicro-structure 1604. The display may also have glass layers and otherlayers, circuitry, and so forth. The display panel 1602 may be amicro-LED display panel. As should be apparent, only one microstructure1604 is depicted for clarity, though a display panel 1602 will have anarray or arrays of microstructures including nanowire LEDs.

The electronic device 1600 may be a mobile device such as smartphone,tablet, notebook, smartwatch, and so forth. The electronic device 1600may be a computing device, stand-alone display, television, displaymonitor, vehicle computer display, the like. Indeed, the electronicdevice 1600 may generally be any electronic device having a display ordisplay panel.

The electronic device 1600 may include a processor 1606 (e.g., a centralprocessing unit or CPU) and memory 1608. The memory 1608 may includevolatile memory and nonvolatile memory. The processor 1606 or othercontroller, along with executable code store in the memory 1608, mayprovide for touchscreen control of the display and well as for otherfeatures and actions of the electronic device 1600.

In addition, the electronic device 1600 may include a battery 1610 thatpowers the electronic device including the display panel 1602. Thedevice 1600 may also include a network interface 1612 to provide forwired or wireless coupling of the electronic to a network or theinternet. Wireless protocols may include Wi-Fi (e.g., via an accesspoint or AP), Wireless Direct®, Bluetooth®, and the like. Lastly, as isapparent, the electronic device 1600 may include additional componentsincluding circuitry and other components.

Thus, embodiments described herein include micro light-emitting diode(LED) display driver architectures and pixel structures.

The above description of illustrated implementations of embodiments ofthe disclosure, including what is described in the Abstract, is notintended to be exhaustive or to limit the disclosure to the preciseforms disclosed. While specific implementations of, and examples for,the disclosure are described herein for illustrative purposes, variousequivalent modifications are possible within the scope of thedisclosure, as those skilled in the relevant art will recognize.

These modifications may be made to the disclosure in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the disclosure to the specific implementationsdisclosed in the specification and the claims. Rather, the scope of thedisclosure is to be determined entirely by the following claims, whichare to be construed in accordance with established doctrines of claiminterpretation.

Example Embodiment 1

A driver circuit for a micro light emitting diode device includes acurrent mirror. A linearized transconductance amplifier is coupled tothe current mirror. The linearized transconductance amplifier is togenerate a pulse amplitude modulated current.

Example Embodiment 2

The driver circuit of example embodiment 1, wherein the current mirrorincludes two P-type transistors.

Example Embodiment 3

The driver circuit of example embodiment 1, wherein the current mirrorincludes four N-type transistors.

Example Embodiment 4

The driver circuit of example embodiment 3, wherein the four N-typetransistors are IGZO-based thin film transistors.

Example Embodiment 5

The driver circuit of example embodiment 1, 2, 3 or 4, wherein thelinearized transconductance amplifier includes a digital to analogconvertor to convert an input voltage to a current.

Example Embodiment 6

The driver circuit of example embodiment 5, wherein the linearizedtransconductance amplifier is to switch the current to the pulseamplitude modulated current.

Example Embodiment 7

The driver circuit of example embodiment 1, 2, 3, 4, 5 or 6, wherein awidth of the pulse of the pulse amplitude modulated current is fixed byan amount of current density needed for representing a maximum Graylevel.

Example Embodiment 8

A backplane of a micro light emitting diode pixel structure includes aglass substrate having an insulating layer disposed thereon. A pluralityof pixel thin film transistor circuits is disposed in and on theinsulating layer. Each of the pixel thin film transistor circuitsincludes a gate electrode and a channel including a semiconducting oxidematerial.

Example Embodiment 9

The backplane of example 8, wherein the semiconducting oxide material isindium gallium zinc oxide (IGZO).

Example Embodiment 10

The backplane of example embodiment 8 or 9, wherein each of the pixelthin film transistor circuits is to drive a single micro light emittingdiode device.

Example Embodiment 11

The backplane of example embodiment 8, 9 or 10, wherein each of thepixel thin film transistor circuits includes a current mirror and alinearized transconductance amplifier coupled to the current mirror.

Example Embodiment 12

The backplane of example embodiment 11, wherein the current mirror ofeach of the pixel thin film transistor circuits includes four N-typetransistors.

Example Embodiment 13

The backplane of example embodiment 11 or 12, wherein the linearizedtransconductance amplifier is to generate a pulse amplitude modulatedcurrent.

Example Embodiment 14

A front plane of a micro light emitting diode pixel structure includes aplurality of micro light emitting diode devices in a dielectric layer. Atransparent conducting oxide layer is disposed above the dielectriclayer. A mask layer is above the transparent conducting oxide layer. Aplurality of microlenses is disposed in openings in the mask layer,individual ones of the plurality of microlenses over a corresponding oneof the plurality of micro light emitting diode devices.

Example Embodiment 15

The front plane of example embodiment 14, wherein the plurality ofmicrolenses includes regions of reflowed photoresist.

Example Embodiment 16

The front plane of example embodiment 14, wherein the plurality ofmicrolenses includes regions of self-assembled microspheres.

Example Embodiment 17

The front plane of example embodiment 14, 15 or 16, wherein thedielectric layer is a carbon-doped oxide layer, the mask includes CrO₂,and the transparent conducting oxide layer is an indium tin oxide (ITO)layer.

Example Embodiment 18

The front plane of example embodiment 14, 15, 16 or 17, wherein theplurality of micro light emitting diode devices includes a single redmicro light emitting diode device, a single green micro light emittingdiode device, and a single blue micro light emitting diode device.

Example Embodiment 19

The front plane of example embodiment 14, 15, 16, 17 or 18, wherein theplurality of micro light emitting diode devices is a plurality ofnanowire-based micro light emitting diode devices.

Example Embodiment 20

The front plane of example embodiment 19, wherein the plurality ofnanowire-based micro light emitting diode devices includes GaNnanowires.

What is claimed is:
 1. A driver circuit for a micro light emitting diodedevice, the driver circuit comprising: a current mirror; and alinearized transconductance amplifier coupled to the current mirror, thelinearized transconductance amplifier to generate a pulse amplitudemodulated current.
 2. The driver circuit of claim 1, wherein the currentmirror comprises two P-type transistors.
 3. The driver circuit of claim1, wherein the current mirror comprises four N-type transistors.
 4. Thedriver circuit of claim 3, wherein the four N-type transistors areIGZO-based thin film transistors.
 5. The driver circuit of claim 1,wherein the linearized transconductance amplifier comprises a digital toanalog convertor to convert an input voltage to a current.
 6. The drivercircuit of claim 5, wherein the linearized transconductance amplifier isto switch the current to the pulse amplitude modulated current.
 7. Thedriver circuit of claim 1, wherein a width of the pulse of the pulseamplitude modulated current is fixed by an amount of current densityneeded for representing a maximum Gray level.
 8. A backplane of a microlight emitting diode pixel structure, the backplane comprising: a glasssubstrate having an insulating layer disposed thereon; and a pluralityof pixel thin film transistor circuits disposed in and on the insulatinglayer, each of the pixel thin film transistor circuits comprising a gateelectrode and a channel comprising a semiconducting oxide material. 9.The backplane of claim 8, wherein the semiconducting oxide material isindium gallium zinc oxide (IGZO).
 10. The backplane of claim 8, whereineach of the pixel thin film transistor circuits is to drive at least onemicro light emitting diode device.
 11. The backplane of claim 8, whereineach of the pixel thin film transistor circuits comprises a currentmirror and a linearized transconductance amplifier coupled to thecurrent mirror.
 12. The backplane of claim 11, wherein the currentmirror of each of the pixel thin film transistor circuits comprises fourN-type transistors.
 13. The backplane of claim 11, wherein thelinearized transconductance amplifier is to generate a pulse amplitudemodulated current.
 14. A front plane of a micro light emitting diodepixel structure, the front plane comprising: a plurality of micro lightemitting diode devices in a dielectric layer; a transparent conductingoxide layer disposed above the dielectric layer; a mask layer above thetransparent conducting oxide layer; and a plurality of microlensesdisposed in openings in the mask layer, individual ones of the pluralityof microlenses over a corresponding one of the plurality of micro lightemitting diode devices.
 15. The front plane of claim 14, wherein theplurality of microlenses comprises regions of reflowed photoresist. 16.The front plane of claim 14, wherein the plurality of microlensescomprises regions of self-assembled microspheres.
 17. The front plane ofclaim 14, wherein the dielectric layer is a carbon-doped oxide layer,the mask comprises CrO₂, and the transparent conducting oxide layer isan indium tin oxide (ITO) layer.
 18. The front plane of claim 14,wherein the plurality of micro light emitting diode devices comprises asingle red micro light emitting diode device, a single green micro lightemitting diode device, and a single blue micro light emitting diodedevice.
 19. The front plane of claim 14, wherein the plurality of microlight emitting diode devices is a plurality of nanowire-based microlight emitting diode devices.
 20. The front plane of claim 19, whereinthe plurality of nanowire-based micro light emitting diode devicescomprises GaN nanowires.